In general, pneumatic tires have a structure such that the crown portion is curved, with the radius of the tread portion growing smaller from the center region towards the shoulder portions.
Due to this difference in radius in the tire width direction, a difference in rolling speed occurs in the tire width direction when the tire is rolling. In particular, the difference in rolling speed becomes pronounced between the center region and the shoulder portions, and the circumferential shear force becomes uneven in the tire width direction.
In particular, a large shear force is generated in the braking direction at the shoulder portions. Due to this shear force, wear progresses in the tread rubber at the shoulder portions, leading to local wear, i.e. uneven wear.
Moreover, when a load is applied to a tire with the above difference in radius, the contact pressure increases at the outside in the width direction, in particular at the contact edge, providing another reason why uneven wear progresses.
Furthermore, as illustrated in FIG. 1, in a pneumatic tire a tread portion 1 and tire side portion 2 are typically joined at an angle. Therefore, the force acting on the tire ground contact area from the tire side portion 2 yields a force pushing the belt 3 inward in the width direction. As a result, a shear force acts on the tread portion 1 in the width direction, causing rubber to undergo shear deformation in the width direction, which is another major reason for wear at the contact edge. This problem is particularly noticeable in tires with a highly rigid side portion, such as run flat tires.
Additionally, out of consideration for stability and wear resistance, the camber is typically aligned in tires, and tires are mounted with a variety of camber angles.
In particular, when the camber angle is large, the above-described force on the belt increases noticeably, thus causing the above uneven wear to progress.
Furthermore, it is also known that when riding a tire with a heavy load or low internal pressure accelerates uneven wear.
To address these problems, Patent Literature 1, for example, discloses a technique for suppressing uneven wear by changing the shape of the tire's crown portion so that the above difference in radius decreases.